Randall K. Bartman

Silicon bulk micromachined vibratory gyroscope for microspacecraft

This paper reports on the design, modeling, fabrication, and characterization of a novel silicon bulk micromachined vibratory rate gyroscope and a 3-axes rotation sensing system using this new type of microgyroscopes designed for microspacecraft applications. The new microgyroscope consists of a silicon four leaf clover structure with a post attached to the center. The whole structure is suspended by four thin silicon cantilevers. This device is electrostatically actuated and detects Coriolis induced motions of the leaves capacitively. A prototype of this microgyroscope has a rotation responsivity (scale factor) of 10.4 mV/deg/sec with scale factor nonlinearity of less than 1%, and a minimum detectable noise equivalent rotation rate of 90 deg/hr, at an integration time of 1 second. The bias stability of this microgyroscope is better than 29 deg/hr. The performance of this microgyroscope is limited by the electronic circuit noise and drift. Planned improvements in the fabrication and assembly of the microgyroscope will allow the use of Q-factor amplification to increase the sensitivity of the device by at least two to three orders of magnitude. This new vibratory microgyroscope offers potential advantages of almost unlimited operational life, high performance, extremely compact size, low power operation, and low cost for inertial navigation and altitude control.

Fabrication and characterization of a micromachined deformable mirror for adaptive optics applications

A novel micromachined electrostatically controlled deformable mirror has been fabricated and characterized. This device combines the fields of microinstruments, adaptive optics and controls to form a silicon-based mirror assembly that is relatively simple to process, inexpensive, lightweight, and integrable with drive and sensing electronics. Electrostatic control of a thin membrane mirror is demonstrated with low voltage actuation and without the need for complex construction of PZT or other translator-type arrays. In addition, the low- stress Si-rich SixNy film used as the deformable membrane mirror is thermally matched to the silicon supporting frame. Custom design of the mirror shape can be implemented by redesigning the electrode pattern on an insulating substrate separate from the thin film mirror. Test results from a pull-only circular mirror with a single actuator are presented as a proof of concept for low voltage actuation of a low-stress SixNy flexible membrane. The SixNy which forms the membrane is under tensile stress. This tensile stress increase the voltage required for deflection of the membrane, but insures a linear relationship between the center deflection of the mirror and the applied pressure. This should significantly simplify the controls algorithm required for closed-loop operation of this device.

Application of the active-pixel-sensor concept to guidance and navigation

Charge-coupled devices (CCDs) have been used extensively in the past in star trackers and fine guidance systems. A new technology, the active pixel sensor, is a possible successor to CCDs. This technology potentially features the same sensitivity and performance of the CCD with additional improvements. These improvements include random access capability, easy window-of- interest readout, non-destructive readout for signal-to-noise improvement, high radiation tolerance, simplified clocking voltages, and easy integration with other on-chip signal processing circuitry. The state-of-the-art of this emerging technology and its potential application to guidance and navigation systems is discussed.

Separated-spacecraft interferometer concept for the New Millennium Program

A separated spacecraft optical interferometer mission concept proposed for NASAs New Millennium Program is described. The interferometer instrument is distributed over three small spacecraft: two spacecraft serve as collectors, directing starlight toward a third spacecraft which combines the light and performs the interferometric detection. As the primary objective is technology demonstration, the optics are modest size, with a 12-cm aperture. The interferometer baseline is variable from 100 m to 1 km, providing angular resolutions from 1 to 0.1 milliarcseconds. Laser metrology is used to measure relative motions of the three spacecraft. High-bandwidth corrections for stationkeeping errors are accomplished by feedforward to an optical delay line in the combiner spacecraft; low-bandwidth corrections are accomplished by spacecraft control with an electric propulsion or cold-gas system. Determination of rotation of the constellation as a whole uses a Kilometric Optical Gyro, which employs counter-propagating laser beams among the three spacecraft to measure rotation with high accuracy. The mission is deployed in a low-disturbance solar orbit to minimize the stationkeeping burden. As it is well beyond the coverage of the GPS constellation, deployment and coarse stationkeeping are monitored with a GPS-like system, with each spacecraft providing both transmit and receive ranging and attitude functions.

Fiber optic rotation sensor for long lifetime space missions

The present F-O rotation sensors (FORS) are all-solid state devices for measuring rotations and rotation rates in inertial space that may reach the 0.003 deg/hr (1-sigma) accuracies required for NASAs Saturn-orbiting Cassini mission. Attention is presently given to the mission, inertial reference unit, and FORS instrument optoelectronic component requirements envisioned for such spacecraft applications.

Polarization dependent effects of radiation on wavelength division multiplexers

The effects of radiation on fused biconical taper wavelength division multiplexers are presented. The polarization sensitivity of these devices before and after irradiation is discussed. Preliminary results on the effects of irradiating different regions of the device, and comparisons between the effects of proton and Co60 radiation sources are also given. A theoretical model that takes into account the index change in the Ge-doped cores of the optical fibers used to make these devices agrees well with experimental observations. This indicates that index changes in the fiber may be primarily responsible for the effects of radiation on these devices.